I live in snow country upstate NY and have two sets of wheels with summer performance tires and winter studless snow tires. We also have long shoulder seasons where the temperature calls for cold weather tires, and there might be some light snow, slush, but not necessarily requiring full blown studdless winters. Over the years it seems that I place a lot of undue wear on my winters where a basic all season would fit better.

I'm considering another set of wheels to create a 3 set system. 1)summer performance, 2) true winter studless (Altimax Arctic or similar), 3) Basic all season tire for shoulder months, October-Jan and March-May?

I dont know how worn your winter tires are, but if you switch to a performance winter tire instead of a studless ice/snow, you will get that functionality without the need for 3 sets and your winter tires will last longer generally since performance winters are designed for cold we/dry road along with snow and ice.

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4-Time NNJR ESP Season Champion -15 ESP
3rd Place in 09' Pax Championship/Driver of the Year
2012 Northeast Divisional Champion in ESP

Previously ran Dunlop 3Ds with great results, go tthem down to the wear bar and pulled them off, put on General Altimax last season for Downstate NY.
loved them has no issues other then slightly loud. (both sets 225/45/17)

Just won a Free set of Blizzak LM-32 in 225/50/17 a little added height will help in larger snow drifts due you lowered ride height from coils.

a third set can be done cheaply, a steelie set or take offs can be found on craigslist and ebay regularly, and a solid all season like cooper Zeon as3 or Toyo extensa hp can had for about $90/ tire

IMO unless you do track days or autocrosses with this car, the current crop of ultra-high performance all-seasons (Michelin Pilot AS3/AS3+, Pirelli P Zero All Season Plus, etc.) are fine for moderately fast/aggressive daily driver duty any time of the year when you wouldn't run a dedicated winter tire. It doesn't snow nearly as much down here so I've made do with one set of wheels with one of the above tires year-round. This class of tires has pretty good steering response and dry grip in the summer, handles rain/downpours without issue, rides decently well on the highway, and they have decent longevity.

If I lived farther north (I lived in Buffalo as a kid) a dedicated set of winters plus a set of UHP all-seasons would cover all the bases for my LGT.

Dedicated performance summer tires (200 treadwear and below), while fun, are of questionable practicality if you're not running HPDE/autox. In daily driver use they'll spend >99% of their time NOT taking advantage of the grip they offer, and they'll ride harder & noisier while wearing out quicker and generally costing you more money. If you do HPDE/autox then yes, 3 sets of wheels & tires makes perfect sense, with as sticky a summer tire as you feel like affording.

I live in snow country upstate NY and have two sets of wheels with summer performance tires and winter studless snow tires. We also have long shoulder seasons where the temperature calls for cold weather tires, and there might be some light snow, slush, but not necessarily requiring full blown studdless winters. Over the years it seems that I place a lot of undue wear on my winters where a basic all season would fit better.

I'm considering another set of wheels to create a 3 set system. 1)summer performance, 2) true winter studless (Altimax Arctic or similar), 3) Basic all season tire for shoulder months, October-Jan and March-May?

Anyone else do this?

For the shoulder months you might also consider an "all-weather" tire. It will have performance a bit more biased towards winter traction than a typical "all-season" tire. If you want even more ice and snow capability (but not as much as a studless winter) a performance winter is another option. Compared to a studless winter it will have better wet and dry traction, and better resistance to hydroplaning and slush planing.